Electroluminescent display device



May 5, 1964 s. YANDO ELECTROLUMINESCENT DISPLAY DEVICE 2 Sheets-Sheet 1Filed June 16, 1960 INVENTOR .S'TEPHE/V YANDU BY H/ J /mmi/ A'ITORNEYUnited States Patent This invention relates to display devices and inparticular to display devices utilizing electroluminescent materials.

This application is a continuation-in-part of mycopending applicationSerial No. 855,419, filed November 25,

1959, wherein I disclose a display device in which a spot of lightis'caused to diagonally traverse an electroluminescent phosphor layeraffixed to one surface of a rectangular sheet of piezoelectric material.The surface of the electroluminescent layer is square and its sides areparallel to and shorter than the corresponding edges of thepiezoelectric sheet. A first electrode, extending along one boundary ofthe electroluminescent layer, is secured to the piezoelectric sheet inthe space between one edge of the sheet and the electroluminescentlayer. Similarly, a second electrode is secured to the piezoelectricsheet in'the space between the adjacent edge of the sheet and theelectroluminescent layer. Thus, the second electrode is perpendicular tothe first electrode. Electrodes connected to a common voltage referencepoint are secured to the other side of the piezoelectric sheet while atransparent electrode is aifixed to the top of the electroluminescentlayer. All four edges of the sheet are provided with terminations whichabsorb, substantially without reflection, any incident elastic wave.

A first voltage pulse applied between the first electrode and thevoltage reference point produces a mechanical strain in thepiezoelectric sheet proportional to, the amplitude of the first pulse.As the strain changes, a disturbance in the from of a first planeelastic wave accompanied by a first electric field, is propagated fromthe first electrode toward the opposite edge of the sheet where it isabsorbed by the termination. The intensity of the electric field isproportional to the time rate of change of the strain that produced it;i.e., the intensity of the first field is proportional to the first timederivative of the first pulse.

Similarly, a second voltage pulse applied between the second electrodeand the voltage reference point produces a second plane elastic Wave.This Wave is accompanied by a second electric field which propagatesfrom the second electrode toward the opposite edge of the sheet where itis also absorbed. The intensity of the second electric eld isproportional to the first time derivative of the second voltage pulse.

Since the first and second electrodes are at right angle to each other,the first and second elastic waves are propagated in mutuallyperpendicular directions. The locus of the intersection of these wavesas they travel through the piezoelectric sheet is a diagonal lineextending across the sheet, the position of the line being a function ofthe difference in time between application of pulses to the first andsecond electrodes. The intensities of the first and second electricfield are additive along the line of intersection and, as a result, abright line of light is produced in the electroluminescent layer alongthe intersection. tween the transparent and grounded electrodes.

While this device is Well suited for scanning a television type imagedisplay, it is sometimes desirable to obtain a higher contrast ratio;i.e., a higher ratio of signal to background illumination. Accordingly,it is an object of this invention to provide an improvedelectroluminescent device in which the contrast ratio is greater thanthat heretofore obtainable.

Modulation is provided by applying a voltage be- Another object of thisinvention is to provide an improved electroluminescent device suitablefor displaying selected discrete points having coordinates correspondingto applied input signals.

Still another object is to provide an improved electroluminescentdisplay device in which the minimum scanning rate does not depend uponthe velocity of propagation though the piezoelectric material.

A further object of the invention is to provide an electroluminescentdisplay device capable of exhibiting continuously varying applied inputsignals.

Yet another object is to provide an electroluminescent display deviceadapted for exhibiting an amplitude modulated signal. i

In the present invention, there is provided a transducing membercomprising a sheet of piezoelectric material having first and secondsurfaces. An electroluminescent layer is affixed to the first surface ofthe sheet and common elec-- trode means are secured to the secondsurface of the sheet.

At least three electrodes are secured to the first surface of thepiezoelectric material, these electrodes being adjacent to theelectroluminescent layer andspaced about its outer perimeter.

In one form of the invention, the sheet of piezoelectric material isrectangular having first, second, third, and fourth edges. First,second, third and fourth electrodes are secured to the first surfaceadjacent the first, second, third and fourth edges respectively. Anelectroluminescent layer having a surface area less than that of thepiezoelectric sheet is afiixed to the sheet in the space encompassed bythe electrodes. The four edges of the sheet are provided withterminations which absorb, substantially without reflection, anyincident elastic wave.

A voltage pulse applied between any one of the first, second, third orfourth electrodes and the common electrode produces a mechanical strainin the piezoelectric sheet proportional to the amplitude of the pulse.The change in strain produces a disturbance in the form of an elasticwave accompanied by an electric field which is propagated at constantspeed toward the opposite edge of the sheet. The intensity of this fieldis proportional to the time rate of change of the strain producing itand, therefore, is proportional to the first time derivative of theapplied pulse.

By applying voltage pulses to all four electrodes simultaneously, fourelastic waves are generated which in tersect in a small region near thecenter of the square electroluminescent layer. The intensities of thefour electric fields produced by the four waves are additive resultingin a bright spot of light at the center of the layer. By adjustin therelative timing of the voltage pulses applied to each of the fourelectrodes, in a manner to be explained hereinafter, any selected areaof the electroluminescent layer may be caused to glow. Further, bycontinuously varying the relative timing of the four pulses, the spot oflight can be caused to scan the panel at an apparent velocity determinedby the timing of the applied pulses.

In another form of the invention, the fourth electrode is omitted, theremaining three electrodes being sufficient to define any point on theelectroluminescent layer. However, while the use of three electrodessimplifies both the display device itself and the necessary excitationequipment, the brightness of the display is somewhat reduced sinceonlythree electric fields intersect at the point instead of four.

The efficiency of the display device is greatly improved amplitudemodulated by applying a signal voltage between the conductive electrodeand the common electrode.

The above objects of and the brief introduction to the present inventionwill be more fully understood and further objects and advantages willbecome apparent from a study of the following description in connectionwith the drawings, wherein:

FIG. 1 is a cut-away perspective view of one embodiment of the displaydevice comprising the invention;

FIG. 2 is a cross sectional view of the display device of FIG. 1;

FIG. 3 is a plan view of the display device of FIG. 1 together with ablock diagram of the excitation circuits; and

FIG. 4'depicts idealized voltage waveforms useful in explaining theoperation of the circuit of FIG. 3.

Referring to FIGS. 1 and 2, there is shown a thin, square, polarized,ceramic piezoelectric sheet consisting of a lead titanate-lead zirconatemixture. An electroluminescent layer 11, having a square surface areasmaller than that of the piezoelectric sheet 10, is placed in intimatecontact with one surface of the sheet. A transparent conductiveelectrode 12 is affixed to the electroluminescent layer 11 while acommon grounded electrode 13 is secured to the opposite side of thepiezoelectric sheet 10. Each edge of the sheet is terminated in suchmanner as to absorb, substantially without reflection, any incidentelastic wave propagated in the sheet. This can be accomplished, forexample, by coating the edges and immediately adjacent portions of sheet10 with lead to provide terminations 14, 15, 16 and 17.

A first linear electrode A extending across the entire width of theelectroluminescent layer 11 is secured to the piezoelectric sheet 10 inthe space between the termination 14 and the side of layer 11.Similarly, linear electrodes B, C, and D are secured to thepiezoelectric sheet 10 in the space between terminations 15, 16, and 17respectively and electroluminescent layer 11.

The application of a voltage pulse between electrode A and groundedelectrode 13 causes a first elastic wave to be propagated across thepiezoelectric sheet 10 at constant speed toward absorbing termination15. This wave is accompanied by an electric field having an intensityproportional to the time rate of change of the pulse applied toelectrode A. A reverse wave also emanates from electrode A but isabsorbed by termination 14 without affecting the display. Similarly,voltage pulses applied to electrodes B, C, and D cause second, third,and fourth elastic waves, accompanied by corresponding electric fields,to be propagated at the same speed as the first wave toward terminations14, 17, and 16 respectively.

When voltage pulses are applied to electrodes A, B, C, and Dsimultaneously, the four elastic waves meet at the center resulting in aspot of light in electroluminescent layer 11 having a brightnessproportional to the total electric field produced by the four waves. Byapplying volt age pulses to the four electrodes A-D at dilferentinstants of time, the four Waves can be made to intersect at anyselected point on the electroluminescent layer.

In FIG. 3 there is shown a plan view of the display device together witha block diagram including a trigger generator 20 and pulse timingcircuit 21. A detailed description of these circuits is provided inco-assigned copending application Serial No. 36,663, filed June 16,1960, by S. Talesnick and, therefore, will not be repeated here. Theelectroluminescent layer 11 together with its conductive film 12 is 2sunits on each side and each electrode is spaced r units from the edge ofthe layer 11. Also, the velocity of propagation of the elastic Wavesthrough the piezoelectric material 10 in all directions has a constantmagnitude v.

Coordinate axes x and y, having their origin at the center of theelectroluminescent layer 11, have been superimposed on the displaydevice to provide a reference grid for defining points on the face ofthe device. In FIG.

3, a point P located x units to the right of the origin and y unitsabove the origin, has been arbitrarily selected as the point to beilluminated.

In order to display the selected point P, the voltage pulses must beapplied to the electrodes A-D at specific instants of time which willpermit the elastic waves propagated through the piezoelectric sheet 10to converge simultaneously upon the point. (Although the illuminatedarea is considered to be a point, it is actually a square having alength' on each side of 30-40 mils or less.) As previously described,the elastic waves are accompanied by electric fields which arecoincident in space with the wave fronts and, therefore, are additive atthe point P; the resultant field producing a spot of light in theelectroluminescent layer at the selected point. Since the electricfields are additive at the point of intersection, a proportionallybrighter spot is obtained using four electrodes than would be obtainedif a lesser number of electrodes were employed.

The application of voltage pulses to the display device is initiated attime i=0 by a timing pulse (FIG. 4a) generated by trigger generator 20and applied to the pulse timing circuit 21. The timing circuit 21 hasfour output leads coupled to electrodes A-D respectively and a pair ofinput terminals 22 and 23. Signal voltages proportional to the desired xand y coordinates are applied to terminals 22 and 23 respectively, thesesignals being used to control the sequence and relative timing of thesignals applied to electrodes A-D in the manner described in theaforementioned copending application Serial No. 36,663. In addition, thex and y signals may be amplitude modulated by applying a modulationvoltage to the terminal 24.

The waveform of the voltage applied to electrode A by pulse timingcircuit 21 is shown in FIG. 4(1)). This voltage comprises a firstsawtooth portion 34) having a steep negative slope and a second sawtoothportion 31 having a gradual positive linear slope. The start of thefirst sawtooth portion 30 of the voltage applied to elec trode A isdelayed by a fixed amount t relative to the timing pulse occurring attime i=0, the interval t being equal to the time required for a wave totravel the distance 2s from one side of the electroluminescent layer 11to the opposite side. This fixed delay is required if the point to bedisplayed is along the left hand side 35 of the electroluminescentlayer. To display such a point, a wave must leave electrode B with suchtiming as to travel the distance r+2s and arrive at edge 35simultaneously with the arrival of the wave from electrode A whichtraveled only the relatively short distance r.

From inspection of FIG. 3, it is seen that the time required for a waveto reach the point P starting from electrode A is equal to while thetime required for a wave to reach the same point starting from electrodeB is only 1/v(r+sx Therefore, the elastic wave must leave electrode B atime interval 2x v later than the wave leaving electrode A. Thus, avoltage pulse is applied to electrode A by timing circuit 21 an intervalt after the timing pulse (t==0) and to electrode B an interval Similarlyit can be shown that pulses must be applied to electrodes C and D attimes and 1:= 1'l' 1) A respectively, to assure that the waves emanatingfrom electrodes A, B, C, and D will arrive at point P simultaneously.

FIGS. 4(b)4(e) depict the voltage waveforms applied to electrodes A Drespectively. The rapidly changing sawtooth voltage shown at St? (FIG.4b) is differentiated in the piezoelectric sheet 10 and, therefore, thevoltage pulse applied to the electroluminescent layer is similar inshape to the trigger pulse shown in FIG. 4a. The duration of the retraceportion 3-1 is equal to 3t this time being required for the waves tocompletely clear the display area regardless of the location of thepoint P. Since the slope of retrace portion 31 is small, its derivativeand, therefore, the voltage applied to the electroluminescent layer isnegligible during the retrace period.

If the input voltages x and y applied to pulse timing circuit 21 varywith time, a curve corresponding to these input voltages may be tracedon the display device. This is accomplished by adjusting the relativetiming of the voltage pulses applied to electrodes A-D for eachconsecutive series of pulses. A series of discrete points will actuallybe obtained on electroluminescent layer 11 but these will appear to theeye as a continuous curve due to the persistance of theelectroluminescent layer 11. The scanning velocity and the intensity ofthe light spot are determined by the amplitudes and relative timing ofthe pulses.

Referring again to FIG. 2, the display device may also be amplitudemodulated by closing switch 36 thereby connecting voltage source 37between the conductive layer 12 and the grounded electrode 13. As theamplitude of voltage source 37 varies, the varying electric fieldproduced in electroluminescent layer 11 by this signal is added to thesummation of the four electric fields produced by application of voltagepulses to elec trodes A-D. Consequently, the light intensities at thepoints of intersection are modulated in accordance with the amplitudevariations of the, source 37.

As many changes could he made in theabove construction and manydifferent embodiments could be made without departing from the scopethereof, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

What is claimed is:

1. A display device comprising a sheet of piezoelectric material havingfirst and second surfaces; an electroluminescent layer having at leastthree edges affixed to the first surface of said sheet; at least threeelectrodes secured to the first surface of said sheet adjacent tocorresponding edges of said electroluminescent layer, said electrodesbeing spaced about said electroluminescent layer, each of saidelectrodes extending substantially along the length of a correspondingedge of said electroluminescent layer; and common electrode meanssecured to the second surface of said sheet.

2. A display device as defined in claim 1 further com prising atransparent conductive electrode afiixed to said electroluminescentlayer.

3. A display device as defined in claim 1, wherein said sheet ofpiezoelectric materialhas at least three edges and wherein terminationmeans are afiixed to each edge of said sheet, said termination meansabsorbing substantially without reflections any incident elastic wavesupplied thereto from said sheet.

4. A display device comprising a sheet of piezoelectric material havingfirst and second surfaces; an electroluminescent layer having first,second, third and fourth edges aflixed to the first surface of saidsheet; first, second, third and fourth electrodes secured to the firstsurface of said sheet adjacent to said first, second, third and fourthedges respectively of said electroluminescent layer, said electrodesbeing spaced about said electroluminescent layer, each of saidelectrodes extending substantially along the length of a correspondingedge of said electroluminescent layer; and common electrode meanssecured to the second surface of said sheet.

5. The display device defined in claim 4 wherein said sheet ofpiezoelectric material and said electroluminescent layer haverectangular cross-sectional areas.

6. A display device as defined in claim 4 wherein said sheetofpiezoelectric material and said electroluminescent layer haverectangular surface areas, the sides of said electroluminescent layerbeing parallel to corresponding edges of said sheet, and wherein saidfirst, second, third, and fourth electrodes are linear and parallel tosaid first, second, third, and fourth edges respectively.

7. A display device as defined in claim 4, wherein said sheet ofpiezoelectric material has first, second, third and fourth edgesadjacent said first, second, third and fourth edges of saidelectroluminescent layer and wherein first, second, third and fourthterminations are affixed to the first, second, third and fourth edges ofsaid sheet, said terminations absorbing substantially withoutreflections any incident elastic wave supplied thereto from said sheet.

8. A display device comprising a sheet of piezoelectric material havingfirst and second surfaces; a rectangular electroluminescent layer havingfirst, second, third, and fourth edges afiixed to the first surface ofsaid sheet; and first, second, third, and fourth linear electrodessecured to the first surface of said sheet adjacent corresponding edgesof said electroluminescent layer, said first, second, third, and fourthelectrodes extending along and being parallel to the first, second,third, and fourth edges of said electroluminescent layer respectively.

9. Apparatus for displaying first and second input voltage signalscomprising in combination, a sheet of piezoelectric material havingfirst and second surfaces and first, second, third and fourth edges; arectangular electroluminescent layer having first, second, third, andfourth edges affixed to the first surface of said sheet; a conductivelayer affixed to said electroluminescent layer; first, second, third,and fourth linear electrodes secured to the first surface of said sheetadjacent the first, second, third and' fourth edges of saidelectroluminescent layer, each of said electrodes extendingsubstantially along the length of a corresponding edge of saidelectroluminescent layer; common electrode means secured to the secondsurface of said sheet; and excitation means coupled to said first,second, third, and fourth electrodes, said excitation means selectivelyapplying pulses to said first, second, third andfourth electrodes andsaid common electrode in response to said first and second input voltagesignals. 1

10. A display device as defined in claim 9 further comprising means forapplying a modulation voltage between said conductive layer and saidcommon electrodemeans.

11. A display device as defined in claim 9, wherein. first, second,third and fourth terminations are afiixed to the first,-second, thirdand fourth edges respectively of said sheet, said'terminations absorbingsubstantially without reflections any incident elastic wave suppliedthereto from said sheet. I

References Cited in the file of this patent UNITED STATES PATENTS YandoAug. 30, 1960

1. A DISPLAY DEVICE COMPRISING A SHEET OF PIEZOELECTRIC MATERIAL HAVINGFIRST AND SECOND SURFACES; AN ELECTROLUMINESCENT LAYER HAVING AT LEASTTHREE EDGES AFFIXED TO THE FIRST SURFACE OF SAID SHEET; AT LEAST THREEELECTRODES SECURED TO THE FIRST SURFACE OF SAID SHEET ADJACENT TOCORRESPONDING EDGES OF SAID ELECTROLUMINESCENT LAYER, SAID ELECTRODESBEING SPACED ABOUT SAID ELECTROLUMINESCENT LAYER, EACH OF SAIDELECTRODES EXTENDING SUBSTANTIALLY ALONG THE LENGTH OF A CORRESPONDINGEDGE OF SAID ELECTROLUMINESCENT LAYER; AND COMMON ELECTRODE MEANSSECURED TO THE SECOND SURFACE OF SAID SHEET.